The present invention relates firstly to a thermal exchange assembly, comprising one or more members of the plate heat-exchanger type which are intended for cooling a gas, secondly to an installation for cooling a gas which employs a thermal exchange assembly according to the invention, and thirdly, and subsidiarily, to a method of cooling a gas which is adapted to make use of a thermal exchange assembly according to the invention.
Because of their large area of exchange surface per unit of volume, plate exchangers, or to be more exact compact plate exchangers made of brazed metal, appear particularly well suited to cooling a gas (whether the gas involved is pure or a mixture of gases), by indirect heat exchange with one or more successive refrigerants (whether the refrigerants have only one constituent or more than one).
However, when one or more multi-constituent refrigerants are used to cool a gas, there is a major, even irremediable, disadvantage in using plate exchangers which results from the need for this refrigerant or these refrigerants to travel in a di-phase form (liquid plus vapour) at some time or other in the cooling cycle. Once this is the case, it is necessary that the liquid and vapour phases of the multi-constituent refrigerant should be uniformly distributed:
possibly between the various heat exchange members, when the latter are arranged in parallel to cool the gas being dealt with. In this regard, given the relatively limited size of plate heat-exchanging members currently available on the market, it is always necessary to use a plurality of members in parallel to cool a gas in large quantities, PA0 between the various passages in the same heat exchange member which are reserved for the flow of the multi-constituent refrigerant, PA0 and within one and the same passage in a heat-exchange member which is reserved for the flow of the said refrigerant, PA0 a plurality of metal plates of substantially similar outline which extend in a first dimension, or length, and a second dimension, or width, and which are spaced from one another and ranged parallel to one another in a third dimension, or thickness, PA0 sealing means which, in conjunction with the aforementioned plates, define a plurality of flattened passages, PA0 at least one passage of a first type which belongs to a first circuit intended for the flow, throughout the length of the member in question, of a first fluid (in particular a refrigerant mixture to be cooled), the sealing means allotted to each passage of the first type leaving open at the two ends of the latter an inlet and an outlet respectively for the refrigerant mixture, PA0 and/or at least one passage of a second type which belongs to a second circuit intended for the flow, over at least a part of the length of the said member, of a second fluid (in particular a gas to be cooled) in co-current with the said first fluid, the sealing means allotted to each passage of the second type leaving open at the two ends of the latter an inlet and an outlet respectively for the said gas, PA0 at least one passage of a third type in thermal exchange relation with at least one of the two passages of the first and second types and belonging to a third circuit intended for the flow, over only a part of the length of the said member, in counter-current with the first and second fluids, of a third fluid (in particular a refrigerant mixture to be heated), the sealing means allotted to each passage of the third type leaving open an inlet and an outlet for the aforementioned mixture, PA0 at least one passage of a fourth type in a thermal exchange relation with at least one of the two passages of the first and second types, belonging to a fourth circuit intended to receive a fourth fluid (in particular an auxiliary refrigerant to be heated), the sealing means allotted to each passage of the fourth type leaving open, at the two ends of the latter, a first opening and a second opening respectively which are reserved for the auxiliary refrigerant, PA0 at least one passage of the fourth type adjacent to a passage of the third type extends over another part of the length of the said member, and at least one transverse partition which extends for the width of the said member separates the two passages respectively of the third and fourth types from one another.
in order to achieve substantially uniform equilibrium temperatures between the liquid and vapour of the multiple refrigerant and thus heat exchange between the said refrigerant and the gas being dealt with which is uniform overall.
The thermodynamic reversibility of the cooling method employed, whatever are the physical operations which are performed successively and cyclically on the multiple refrigerant, and thus the attainment of a satisfactory energy efficiency for the method selected, are achieved at the expense of having the multiple refrigerant in di-phase form in the course of cooling, and/or while it is heating up, and/or before it is heated up.
To distribute a di-phase fluid (liquid plus gas) uniformly between the various passage in one and the same plate exchanger, various arrangements have already been proposed but none of these has proved satisfactory, either because they result in unacceptable technical complexity or because the uniformity achieved in the di-phase distribution is still unsatisfactory.
Starting from this realisation, in accordance with the present invention and in contrast to solutions proposed in the prior art, an attempt has been made to solve the problem described above by restricting the need for and the extent of di-phase distribution in a plate exchanger to the minimum, not only as regards the multi-constituent refrigerants used but also as regards the gas to be cooled, and this has been done by using particular arrangements in the exchanger or exchangers employed, and/or by selecting particular conditions of operation in the cooling cycle or cycles selected.